Dissolved Barium (Ba) from R/V Thomas G. Thompson cruise TN303 (GP16) in the Eastern Tropical Pacific in 2013 (U.S. GEOTRACES EPZT project) (NCEI Accession 0278725)

Acquisition Description:
Water column samples were taken from the GEOTRACES carousel and filtered through pre-cleaned, 0.2 um Pall Acropak Supor filter capsules as described elsewhere (e.g., Cutter et al., 2012; Hatta et al., 2015). Near surface water samples were collected using an underway towed-fish pumped seawater system (Bruland et al., 2005) with samples filtered through sequential 0.45 um Osmonics and 0.2 µm Polycarbonate (PCTE) cartridge filters. Filtered water was collected in 125 mL HDPE bottles (Nalgene) that had been precleaned by soaking in hot 1.2 M HCl (reagent grade) for at least 8 h with subsequent thorough rinsing with ultrapure distilled deionized water (Barnstead E-pure). Samples were acidified in a laminar flow bench aboard ship using 0.5 mL of ultrapure HCl per 125 mL sample.

Barium was measured using a ThermoFisher Element 2 Inductively Coupled Plasma Mass Spectrometer (ICP-MS) and the isotope dilution method as described by Jacquet et al. (2005). Aliquots (50 uL) of each sample were spiked with 25 uL of a 135Ba-enriched solution (~170 nM) and then diluted 30-fold with 0.2 um ultrapure filtered water. A sample of ~93% enriched 135Ba was obtained from Oak Ridge National Laboratories for use as the enriched isotope spike. The ICP-MS was operated in low resolution and both 135Ba and 138Ba were determined. The samples were bracketed every 10 samples with a blank and the spike 135Ba solution. The volumes of the spikes, samples and dilution water were accurately assessed by calibrating each pipette by weight. The reproducibility error of this method was estimated by comparing samples collected at the same depths on different casts at the same station. For 42 pairs of these replicate samples, the average absolute deviation of 1.4 nmol/kg or typically 1.5%. Repeated runs of runs of US GEOTRACES intercalibration samples and in-house reference solutions suggest a precision of ±2.0%; the limit of detection for barium was 0.7 nmol/kg. Our precision is similar to that reported by other labs for Ba (e.g., Jacquet et al., 2005).